There has been known a trochoid type oil pump including a housing (gear case), an outer rotor which has internal teeth as being rotatably arranged in the housing, an inner rotor which has external teeth engaged with the internal teeth of the outer rotor and which defines a volume-varying pump chamber in cooperation with the outer rotor, a rotary shaft which is rotatably supported by the housing to rotate the inner rotor, two side plates which are capable of being in contact with both side faces of the inner rotor and the outer rotor in the axis line direction of the rotary shaft and being moved in the axis line direction with slight clearance formed in the axis line direction, two elastic members which are arranged in the housing to press the two side plates toward both the side faces of the inner rotor and the outer rotor, and the like. Here, even in a case that dimensional variation in the axis line direction occurs at the housing, the inner rotor, and the outer rotor with thermal expansion and the like, the elastic members continuously press the two side plates respectively to both side faces of the inner rotor and the outer rotor. Accordingly, stable volume efficiency can be obtained without causing clearance (for example, see Patent Literature 1).
The abovementioned oil pump adopts a structure that the two non-rotatable side plates are pressed directly to the rotating inner rotor and outer rotor. Therefore, slide resistance becomes large, so that large rotational torque is required to operate the oil pump. Consequently, operational load of an engine or the like is increased.
Further, the two side plates are relatively slid in a state of being continuously pressed to both the side faces of the inner rotor and the outer rotor at predetermined pressure. Therefore, in a case that the side plates are made of softer material than that of the inner rotor and the outer rotor, wear, deterioration with time, and the like are more likely to occur to cause a problem in durability.